Refinement of the ferrite grain size has led to improvement in the strength and toughness of steels. The final ferrite grain size of the steel can be determined, in large part, by the austenite grain size prior to cooling and transformation to ferrite grains. However, austenite grain growth also occurs during the processing of the steel, e.g., during hot rolling, thermomechanical processing, normalizing, welding, enamelling or annealing. If coarse austenite grains are formed during such processing, they are often difficult to refine in subsequent processing operations, and such refinement comes at added cost in processing of the steel. Coarsening of austenite grains during processing can result in the steel having poor mechanical properties.
Steels containing a fine dispersion of small stable particles as those found in Al, Ti, Nb and V steels have been shown in the past to resist austenite grain growth at high temperature. The elements form stable nitrides, carbides and/or carbonitride precipitates in the steel that resist austenite grain growth at high temperatures. The ability of these particles to resist dissolution and coarsening, in the past, has been considered essential in resisting austenite grain growth at high temperatures.
This invention relates to carbon steel products that exhibit a high austenite grain coarsening temperature, without the necessity for additions of conventional austenite grain refining elements such as Al, Nb, Ti, and V. These elements form nitride or carbo-nitride particles, which act to provide a high austenite grain coarsening temperature, whereas the steel of this invention utilizes precipitated, fine oxide particles comprising Si, Fe and O to achieve similar high austenite coarsen temperatures. The steel composition presently disclosed has high levels of oxygen and a dispersion of silicon and iron oxide particles of less than 50 nanometers and generally ranging from ranging in size from 5 to 30 nanometers.
The ability to restrict austenite grain growth during heat treatment cycles and welding processes facilitates the achievement of a fine final microstructure on cooling to ambient temperature. A high austenite grain coarsening temperature provides a wide temperature range from which a known and reliable austenite grain size will be produced, which aids in achieving the desired final microstructure. In the case of a low carbon steel presently disclosed, cooled under air cooling conditions, the resultant fine ferrite grain size is conducive to achieving an attractive combination of strength, toughness and formability.
The steel product presently disclosed also exhibits a high ferrite recrystallization temperature. Such an attribute can restrict or even prevent the extent of critical strain grain growth of ferrite. This phenomenon induced through heating lightly plastically strained areas in cold formed steel products to subcritical temperatures. The resultant large ferrite grain size can provide a low strength region in the formed product, which maybe deleterious to the performance of the product. At low strain levels the nucleation rate of new recrystallised ferrite grain size is low, which leads to the growth of large ferrite grains.
The steel product of the present invention may be made by continuous casting strip steel in a twin roll caster. In twin roll casting, molten metal is introduced between a pair of counter-rotated horizontal casting rolls, which are cooled, so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting thin steel strip in a twin roll caster, the molten steel in the casting pool will generally be at a temperature of the order of 1500 to 1600° C., and above, and therefore high cooling rates are needed over the casting roll surfaces. It is important to achieve a high heat flux and extensive nucleation on initial solidification of the steel on the casting surfaces to form the metal shells during casting. U.S. Pat. No. 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry so that a substantial proportion of the metal oxides formed as deoxidation products are liquid at the initial solidification temperature so as to form a substantially liquid layer at the interface between the molten metal and the casting surface. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, nucleation of the steel on initial solidification can be influenced by the texture of the casting surface. In particular International Application AU 99/00641 discloses that a random texture of peaks and troughs can enhance initial solidification by providing potential nucleation sites distributed throughout the casting surfaces. We have now determined that nucleation is also dependent on the presence of oxide inclusions in the steel melt and that, surprisingly, it is not advantageous in twin roll strip casting to cast with “clean” steel in which the number of inclusions formed during deoxidation has been minimized in the molten steel prior to casting. We have found that the extremely high cooling rates result in high levels of oxygen in the steel composition and the formation of a fine precipitated dispersion of silicon and iron oxide particles of less than 50 nanometers and generally ranging in size from 5 to 30 nanometers. The composition of these particles we believe to be Si—Fe—O spinel.
Steel for continuous casting is subjected to deoxidation treatment in the ladle prior to pouring. In twin roll casting, the steel is generally subjected to silicon manganese ladle deoxidation. However, it is possible to use aluminum deoxidation with calcium addition to control the formation of solid Al2O3 inclusions that can clog the fine metal flow passages in the metal delivery system through which molten metal is delivered to the casting pool. It has hitherto been thought desirable to aim for optimum steel cleanliness by ladle treatment and minimize the total oxygen level in the molten steel. However, we have now determined that lowering the steel oxygen level reduces the volume of inclusions, and if the total oxygen content and free oxygen content of the steel are reduced below certain levels the nature of the intimate contact between the molten steel and casting roll surfaces can be adversely effected to the extent that there is insufficient nucleation to generate rapid initial solidification and high heat flux. Molten steel is trimmed by deoxidation in the ladle such that the total oxygen and free oxygen contents fall within ranges which ensure satisfactory solidification on the casting rolls and production of a satisfactory steel strip. The molten steel contains a distribution of oxide inclusions (typically MnO, CaO, SiO2 and/or Al2O3) sufficient to provide an adequate density of nucleation sites on the casting roll surfaces for initial and continued high solidification rates and the resulting strip product exhibits a characteristic distribution of solidified inclusions and surface characteristics.
We have produced a steel product with a high austenite grain coarsening temperature comprising less than 0.4% carbon, less than 0.06% aluminium, less than 0.01% titanium, less than 0.01% niobium, and less than 0.02% vanadium by weight and having fine-size oxide particles containing silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers in size, or less than 40 nanometers in size. The average oxide particle size may be between 5 and 30 nanometers. The aluminium content may be less than 0.05% or 0.02% or 0.01%. The molten steel used to produce the steel product may include oxide inclusions comprising any one or more of MnO, SiO2 and Al2O3 distributed through the steel at an inclusion density in the range 2 gm/cm3 to 4 gm/cm3. The oxide inclusions in the molten steel may range in size between 2 and 12 microns.
The steel product with a high austenite grain coarsening temperature may comprise less than 0.4% carbon, less than 0.06% aluminium, less than 0.01% titanium, less than 0.01% niobium, and less than 0.02% vanadium by weight and having fine-size oxide particles capable of producing austenite grains through the microstructure resistant to coarsening at high temperature. The steel microstructure has an average austenite grain size of less than 50 microns, or less than 40 microns, up to at least 1000° C., or even greater than 1050° C., for a holding time of at least 20 minutes. The average austenite grain size may be between 5 and 50 microns up to least 1000° C., or at least 1050° C., for a holding time of at least 20 minutes. The fine particles may be oxides of silicon and iron less than 50 nanometers in size. The aluminium content may be less than 0.05% or 0.02% or 0.01% by weight.
Alternatively, the steel product with a high austenite grain coarsening temperature is a carbon steel of less than 0.4% carbon, less than 0.06% aluminium, less than 0.01% titanium, less than 0.01% niobium, and less than 0.02% vanadium by weight may be capable of resisting ferrite recrystallization up to temperatures of 750° C. for strain levels up to at least 10% (for conventional processing heating rates and holding times up to at least 30 minutes) The steel product with a high austenite grain coarsening temperature may have a carbon content less than 0.01%,or less than 0.005%, and aluminium content less than 0.01% or less than 0.005%.
The steel product with a high austenite grain coarsening temperature may be made in a twin roll caster with the molten steel having total oxygen content in the casting pool of at least 70 ppm, usually less than 250 ppm, and a free-oxygen content of between 20 and 60 ppm. The molten steel may have total oxygen content in the casting pool of at least 100 ppm, usually less than 250 ppm, and a free-oxygen content between 30 and 50 ppm. The closely controlled chemical composition of the molten steel, particularly the soluble oxygen content, and the very high solidification rate of the process, provide conditions for the formation of fine-sized, generally spheroid-shaped oxide particles distributed through the steel microstructure, which restrict the average austenite grain size, on subsequent reheating to less than 50 microns for temperatures up to least 1000° C. for a holding time of at least 20 minutes.
The austenite grain coarsening properties exhibited by the present steel product are similar to or better than those generally observed with conventional normalized aluminium killed steels, where the presence of aluminium nitride particles in the steel microstructure act to restrict austenite grain growth. The austenite grain coarsening properties of the steel in fact approach the grain coarsening properties observed with titanium treated aluminium killed continuously slab cast steels. See, JP Publication No. S61 [1986]-213322. In titanium treated aluminium killed steels, the cooling rates of continuously cast slabs produces fine titanium nitride particles, with particle sizes ranging down to 5-10 nanometers. The ability of aluminium to form a suitable dispersion of aluminium nitride particles when the appropriate levels of aluminium and nitrogen are present in the steel has lead to the production of aluminium killed fine-grained steels. However, in the case of strip steels produced via hot strip mills, the high cooling rates of the steel strip through the temperature range in which aluminium nitride particles precipitate, during post rolling cooling processes, can limit the extent of the precipitation. (For conventional coiling temperatures of less than about 700° C.) This can be particularly evident at strip edges and coil ends even at aluminium levels over 0.02% and up to 0.06%. Furthermore, the high heating rates typically achieved on the subsequent reheating of strip steels also restricts the extent of aluminium nitride precipitation. Hence aluminium killed strip steels may not necessarily exhibit a high austenite grain coarsening temperature. For the steel product of this invention, the cooling rate of the strip during post rolling cooling processes, does not substantially affect the austenite grain coarsening temperature of the steel.
The presently described steel product with a high austenite grain coarsening temperature has a microstructure with austenite grain growth inhibition better than aluminium killed fine grained steels in the absence of the conventional grain refining elements, aluminium, titanium, niobium and vanadium. Unique steel with different microstructure and resulting strength properties is thus provided by the present cast steel, and without the added costs associated with such fine grained steels in the past. The austenite grain coarsening properties of the present cast steel confers benefits as refinement of the microstructure of the heat affected zone associated with welding processes and other heat treatments such as normalizing, enamelling and annealing. In the past, excessive coarsening of austenite grains during heat treatment has been found to lead to coarse microstructure in the steel on cooling and an associated loss of strength and toughness in the steel at ambient temperatures.
Note that the titanium, niobium and vanadium levels in the presently disclosed steel products are generally those observed as impurities introduced by using scrap as a starting material for making the steel in an electric arc furnace. However, purposeful introduction of titanium, niobium and vanadium may be made without avoiding the presently claimed invention where the levels are so low that they do not provide the fine grain features by alternative means as discussed above.
A low carbon steel strip with a high austenite grain coarsening temperature may be made by the steps comprising:                assembling a pair of cooled casting rolls having a nip between them and confining closures adjacent the ends of the nip;        introducing molten carbon steel between said pair of casting rolls to form a casting pool between the casting rolls with said closures confining the pool adjacent the ends of the nip, with the molten steel having a total oxygen content in the casting pool of at least 70 ppm, usually less than 250 ppm, and a free-oxygen content of between 20 and 60 ppm;        counter rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls with levels of oxide inclusions reflected by the total oxygen content of the molten steel to promote the formation of thin steel strip; and        forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.        
A carbon steel strip with a high austenite grain coarsening temperature may also be made by the step comprising:                assembling a pair of cooled casting rolls having a nip between them and confining closures adjacent the ends of the nip;        introducing molten carbon steel between said pair of casting rolls to form a casting pool between the casting rolls with said closures confining the pool adjacent the ends of the nip, with the molten steel having a total oxygen content in the casting pool of at least 100 ppm, usually less than 250 ppm, and a free-oxygen content between 30 and 50 ppm;        counter rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls with levels of oxide inclusions reflected by the total oxygen content of the molten steel to promote the formation of thin steel strip; and        forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.        
The total oxygen content of the molten steel in the casting pool may be about 200 ppm or about 80-150 ppm. The total oxygen content includes free oxygen content between 20 and 60 ppm or between 30 and 50 ppm. Note, the free oxygen may be measured at a temperature between 1540° C. and 1600° C., which is the typical temperature of the molten steel in the metal delivery system where the oxygen content is typically measured. The total oxygen content includes, in addition to the free oxygen, the deoxidation inclusions present in the molten steel at the introduction of the molten steel into the casting pool. The free oxygen is formed into solidification inclusions adjacent to the surface of the casting rolls during formation of the metal shells and cast strip. These solidification inclusions are liquid inclusions that improve the heat transfer rate between the molten metal and the casting rolls, and in turn promote the formation of the metal shells. The oxidation inclusions also promote the presence of free oxygen and in turn solidification inclusions, so that the free oxygen content is related to the oxidation inclusion content.
The low carbon steel here is defined as steel with a carbon content in the range 0.001% to 0.1% by weight, a manganese content in the range 0.01% to 2.0% by weight and a silicon content in the range 0.20% to 10% by weight. The steel may have aluminum content of the order of 0.02% or 0.01%, or less, by weight. The aluminum may for example be as little as 0.008% or less by weight. The molten steel may be a silicon/manganese killed steel.
The oxide inclusions are solidification inclusions and deoxidation inclusions. The solidification inclusions are formed during cooling and solidification of the steel in casting, and the oxidation inclusions are formed during deoxidation of the molten steel before casting. The solidified steel may contain oxide inclusions usually comprised of any one or more of MnO, SiO2 and Al2O3 distributed through the steel at an inclusion density in the range 2 gm/cm3 and 4 gm/cm3.
The molten steel may be refined in a ladle prior to introduction between the casting rolls to form the casting pool by heating a steel charge and slag forming material in the ladle to form molten steel covered by a slag containing silicon, manganese and calcium oxides. The molten steel may be stirred by injecting an inert gas into it to cause desulphurization, and then injecting oxygen, to produce molten steel having the desired total oxygen content of at least 70 ppm, usually less than 250 ppm, and a free oxygen content between 20 and 60 ppm in the casting pool. As described above, the total oxygen content of the molten steel in the casting pool may be at least 100 ppm and the free oxygen content between 30 and 50 ppm. In this regard, we note that the total oxygen and free oxygen contents in the ladle are generally higher than in the casting pool, since both the total oxygen and free oxygen contents of the molten steel are directly related to its temperature, with these oxygen levels reduced with the lowering of the temperature in going from the ladle to the casting pool. The desulphurization may reduce the sulphur content of the molten steel to less than 0.01% by weight.
The thin steel strip produced by continuous twin roll casting as described above has a thickness of less than 5 mm and is formed of cast steel containing solidified oxide inclusions. The distribution of the inclusions in the cast strip may be such that the surface regions of the strip to a depth of 2 microns from the outer faces contain solidified inclusions to a per unit area density of at least 120 inclusions/mm2.
The solidified steel may be a silicon/manganese killed steel and the oxide inclusions may comprise any one or more of MnO, SiO2 and Al2O3 inclusions. The inclusions typically may range in size between 2 and 12 microns, so that at least a majority of the inclusions are in that size range.
The method described above produces a unique steel high in oxygen content distributed in oxide inclusions. Specifically, the combination of the high oxygen content in the molten steel and the short residence time of the molten steel in forming steel strip has resulted in unique steel with improved ductility and toughness properties.